The present invention relates to a green light emitting phosphor that emits green light upon excitation with long-wave ultraviolet (UV) radiation or visible light of 350 to 500 nm. The present invention also relates to a light emitting device using the phosphor.
Light emitting diodes (LEDs) are semiconductor light emitters adapted to produce light by converting electrical energy into ultraviolet light, visible light, infrared light or the like. For example, those emitters utilizing visible light are semiconductor light emitters formed from light emitting materials such as GaP, GaAsP and GaAlAs, and LED lamps having such emitters encapsulated with transparent resins are widely used. Also LED lamps of the display type are often used in which light emitting materials are secured to an upper surface of a printed circuit board or metal leads and encased in a transparent resin casing on which numerical figures or characters are configured.
LEDs have a long lifetime and high reliability because of semiconductor devices, and facilitate replacement operation when used as light sources. LEDs are thus widely used as components in portable communications equipment, personal computer peripheral equipment, business machines, household electric appliances, audio equipment, switches, back light sources, and display device such as bulletin boards. Moreover, LED lamps are attracting attention for use as the backlight illuminants of on-vehicle displays for such as meters, heater control panels and audio displays. There has been an increasing demand for LED lamps that intensely emit light, particularly in the white light or blue-green wave region light.
The color of light emitted by the LED lamps can be altered by introducing various powder phosphors into transparent resins with which semiconductor light emitters are encapsulated. Depending on the intended application, any color in a wide spectrum in the visible region from blue to red is available. However, conventional green light phosphors used for fluorescent lamps sash as three-band fluorescent lamps are satisfactory in emission of short wave UV radiation but unsatisfactory in emission of long wave UV radiation or visible light of 350 to 500 nm for high luminance LED lamps. These conventional green light phosphors emit light due to transition from 5D4 to 7F5 of Tb3+ ions typified by LaPO4:Ce3+, Tb3+ and CeMgAl11O19:Tb3+.
Recent users increasingly impose a more stringent demand on the color of such various display devices, requiring display devices to have an ability to precisely reproduce a subtle color tone. It is strongly required that a single LED lamp emits white or any desired intermediate color light. Attempts were then made to display white or any desired intermediate color with a single LED lamp by applying various red, green and blue phosphors to the surface of the semiconductor light emitter in the LED lamp, or incorporating such various phosphors in the encapsulant or coating material of the LED lamp.
The conventional green light emitting phosphor, however, does not produce high luminance white light when it is excited with long wave UV radiation or visible light of 350 to 500 nm, due to deviation in luninous efficiency of the resulting the green light.
As these phosphors which emit light upon excitation with long wave UV radiation or visible light, there have been developed various emitting phosphors, which are excited with long wave UV radiation or short wave visible light (350 to 420 nm). Examples of the green light emitting phosphors are BaMg2Al16O27:Eu,Mn and Zn2GeO4:Mn. Examples of the blue light emitting phosphors are BaMg2Al16O27:Eu and (Sr,Ca,Ba)5(PO4)3Cl:Eu. Examples of the red light emitting phosphors are Y2O2S:Eu, La2O2S:Eu, and 3.5 MgO.0.5 MgF2.GeO2:Mn. The combined use of these phosphors produces light over a broad range of color. However, for development of practical LED lamps capable of emitting high luminance white light, it is necessary to develop new phosphors for higher luminance and better luninous efficiency. Incidentally, these green light emitting phosphors does not achieve high luminance because the green light emitted from the green light emitting phosphors considerably deviates from 555 nm that gives a good luninous efficiency.
On the other hand, it would be possible to obtain a white light emitting LED lamp by combination of an LED that emits blue to bluish green visible light (430 to 500 nm) and a phosphor (such as YAG:Ce3+) that emits yellow light upon excitation with the light. Unfortunately, this idea has not yet been realized because there are no phosphors available which emit green light upon excitation in the region of wavelengths mentioned above.